One Flood Is Not Enough: Pool-Riffle Self-Maintenance Under Time-Varying Flows and Nonequilibrium Multifractional Sediment Transport

The interaction of sediment supply and hydrographs can affect, on a mesoscale, geomorphic features like pools and riffles, which are fundamental units of many gravel bed rivers. In the past decades, different hypotheses have been developed to characterize the hydrodynamics of pool-riffle sequences; however, most of the previous studies considered equilibrium or near-equilibrium sediment transport conditions. Here we investigate the stability of pools and riffles during a sequence of different hydrographs representative of a natural flow regime, without satisfying the equilibrium sediment transport condition. In the current study, the effects of bed geometry, sediment sorting and hydrograph duration, are explained and quantified. The results show that under nonequilibrium conditions, the reversal episodes are not always competent enough for complete self-maintenance during a single flood. However, width variations and grain sorting effects prevented the pools to be completely filled up with the upstream sediment supply. Hydrograph duration had a significant role in the riffle bed geometry. Even though a single flood (irrespective of the magnitude) was not competent enough to restore the pool-riffle feature, a sequence of floods progressively improved conditions for self-maintenance. These findings can bring more insight into flow management strategies, in terms of the importance of multiple sequential floods for restoring rivers with high sediment supply.